Suspensions of solid particles in liquids are frequently used in technical fields--for example, suspensions of barium sulphate in liquid as contrast medium for X-ray examination equipment, magnetizable, fluorescent particles in liquids for crack testing; lubricating oils, which as time goes on acquire an increased solid content of metallic fines, abrasive suspensions, etc.
Many of these suspensions are circulated in systems--for example, as lubricants or even the fluorescent fluid in crack testing equipment. The ability of the test equipment to make a statement depends, amongst other things, on these suspensions being intact--if there are too few solid particles or if they have been damaged by mechanical stress, the test results obtained with them are often poor or no longer usable. The opposite case also exists where, for example, an increased input of solids into a liquid and therefore the formation of an ever increasingly concentrated suspension has to be checked, so that a statement can be made regarding the continued suitability of the medium--for example, a lubricating oil--in order to prevent damage to an engine. On the other hand it is desirable, both from the environmental and cost points of view, to use the suspension for as long as possible, in order to avoid unnecessary disposal.
Actually most engine oils and also most crack detecting agents are changed after a pre-determined "service interval", as a preventive measure--although for many applications this should not be necessary. On the other hand increased stress on the suspension--for example, from the action of particularly high shearing forces on the liquid and also when working at high temperature--the suspension may degrade more quickly or be "used up".
Hitherto such suspensions, in the case of conductive materials, were often tested by way of conductivity tests (for retarding agents), pH value measurements or even by sedimentation and visual observation thereof in a so-called ASTM bulb. The conductivity test had the disadvantage that even a slight change in the electrolyte or water content of oil, for example, led to completely incorrect measurements--the ASTM bulb had the serious drawback of up to now delivering values that were not automatically acquirable and could therefore only be measured subjectively.
From EP-A-O 427 996 discloses a process for measuring aromatic hydrocarbons, where fluorescence spectroscopy is used to measure the concentration of fluorescent, i.e. liquid-dissolved hydrocarbons. In other words, in this case, clear liquids and not suspensions are being measured.
DE-A-43 11 543 relates to a device for determining the concentration of a test liquid, which only measures the luminescence, i.e. the light emitted by the test liquid but not whether this lunescence may possibly change in time and therefore does not permit any statement to be made as to whether the luminescence occurs as a result of abraded fluorescent material or as a result of the particles themselves, to which the fluorescrible material is bound. This document also does not enable any measurements to be made of the solid density of the suspension, since the emission of fluorescent light does not enable any such statements to be made.
JP-A-62-255851 in turn, relates to the measurement of the rate of sedimentation of a material, as required for coagulation measuring devices, for example for the solvent coagulation of plastics. In other words, the increase in size of the suspended particles is measured over time. The area of the invention on the other hand, relates to the measurement of fluorescent material, which over a period of time, also decreases in size.